1. Field of the Invention
The present invention relates to an exhaust gas purification device for an internal combustion engine. More specifically, the invention relates to a device which is capable of removing NO.sub.x from the exhaust gas of a lean burn engine with high efficiency.
2. Description of the Related Art
An exhaust gas purification device utilizing a three-way reducing and oxidizing catalyst (hereinafter referred to as a "three-way catalyst") is commonly used for removing HC, CO and NO.sub.x from the exhaust gas of an internal combustion engine (in this specification, the term NO.sub.x means a nitrogen oxide such as NO, NO.sub.2, N.sub.2 O and N.sub.2 O.sub.4, in general). The three-way catalyst is capable of oxidizing HC and CO, and reducing NO.sub.x, in the exhaust gas when the exhaust gas is at a stoichiometric air-fuel ratio. Namely, the three-way catalyst is capable of simultaneously removing these harmful compounds from exhaust gas when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio.
However, the ability of the three-way catalyst for reducing NO.sub.x becomes lower as the air-fuel ratio of the exhaust gas becomes leaner (i.e., as the air-fuel ratio becomes higher than the stoichiometric air-fuel ratio). Therefore, it is difficult to remove NO.sub.x in the exhaust gas from a lean burn engine, which is operated, on the whole, at a lean air-fuel ratio, using a three-way catalyst.
To solve this problem, Japanese Unexamined Patent Publication (Kokai) No. 4-365920 discloses an exhaust gas purification device utilizing a denitrating reaction.
When the air-fuel ratio of the exhaust gas is lower than the stoichiometric air-fuel ratio (i.e., when the air-fuel ratio of the exhaust gas is rich), the three-way catalyst converts a portion of NO.sub.x in the exhaust gas to NH.sub.3 while reducing most of NO.sub.x in the exhaust gas and converting it into N.sub.2. The device in the '920 publication produces NH.sub.3 from NO.sub.x in the exhaust gas using a three-way catalyst, and reacts the produced NH.sub.3 with the NO.sub.x in the exhaust gas to reduce NO.sub.x to N.sub.2 and H.sub.2 O by a denitrating reaction.
In the '920 publication, a multi-cylinder internal combustion engine is used, and a group of cylinders of the engine are operated at a rich air-fuel ratio while other cylinders are operated at a lean air-fuel ratio, and the operating air-fuel ratio of the engine, as a whole, is kept at a lean air-fuel ratio. Further, a three-way catalyst having a high capability for converting NO.sub.x to NH.sub.3 is disposed in an exhaust gas passage connected to the rich air-fuel ratio cylinders (i.e., the cylinders operated at a rich air-fuel ratio). After it flows through the three-way catalyst, the exhaust gas from the rich air-fuel ratio cylinders mixes with the exhaust gas from the lean air-fuel ratio cylinders. When the exhaust gas from the rich air-fuel ratio cylinders flows through the three-way catalyst, a portion of the NO.sub.x in the exhaust gas is converted to NH.sub.3. Thus, the exhaust gas downstream of the three-way catalyst contains a relatively large amount of NH.sub.3. On the other hand, the exhaust gas from the lean air-fuel ratio cylinders contains a relatively large amount of NO.sub.x. Therefore, by mixing the exhaust gas from the three-way catalyst and the exhaust gas from the lean air-fuel ratio cylinders, NH.sub.3 in the exhaust gas from the three-way catalyst reacts with NO.sub.x in the exhaust gas from the lean air-fuel ratio cylinder, and NH.sub.3 and NO.sub.x produce N.sub.2 and H.sub.2 O by a denitrating reaction. Thus, according to the device in the '920 publication, NO.sub.x is removed from the exhaust gas.
In the device of the '920 publication, it is required that the amount of NH.sub.3 produced by the three-way catalyst is sufficient for reducing all of the NO.sub.x in the exhaust gas from the lean air-fuel ratio cylinders. For example, the greatest part of NO.sub.x in the exhaust gas discharged from the engine is composed of NO (nitrogen monoxide) and NO.sub.2 (nitrogen dioxide) components. These NO and NO.sub.2 components react with NH.sub.3 and produce N.sub.2 and H.sub.2 O by the following denitrating reactions. EQU 4NH.sub.3 +4NO+O.sub.2 .fwdarw.4N.sub.2 +6H.sub.2 O EQU 8NH.sub.3 +6NO.sub.2 .fwdarw.7N.sub.2 +12H.sub.2 O
Therefore, in the device of the '920 publication, an amount of NH.sub.3 which equals the total of the number of moles of NO and 4/3 times the number of moles of NO.sub.2 is required in order to remove all of the NO.sub.x in the exhaust gas from the lean air-fuel ratio cylinders. When the exhaust gas contains other NO.sub.x components such as N.sub.2 O.sub.4, N.sub.2 O, the amount of NH.sub.3 stoichiometrical to the amount of these components is required in addition to the above noted amount on NH.sub.3.
However, the amount of NO.sub.x produced in the cylinders of the engine becomes the maximum when the cylinders are operated at a lean air-fuel ratio (for example, at an excess air ratio about 1.2), and decreases rapidly when the cylinders are operated at a rich air-fuel ratio. Since the device in the '920 publication converts NO.sub.x in the exhaust gas of the rich air-fuel ratio cylinder to produce NH.sub.3, the amount of produced NH.sub.3 is limited by the amount of NO.sub.x produced in the rich air-fuel ratio cylinders. Therefore, in the device of the '920 publication, the amount of NH.sub.3 produced by the three-way catalyst is not sufficient to reduce all of the NO.sub.x in the exhaust gas from the lean air-fuel ratio cylinders, and a part of NO.sub.x in the exhaust gas from the lean air-fuel ratio cylinder is released to the atmosphere without being reduced.
Further, in the device of the '920 publication, a group of the cylinders are operated at a lean air-fuel ratio, while other cylinders of the engine is operated at a rich air-fuel ratio. This causes a difference in the output torque of the cylinders, and causes fluctuations in the output torque of the engine.